A lighting device that causes LEDs (also called a light-emitting diode) to light up by a voltage waveform in the shape of a pulsating current or in the shape close to a pulsating current obtained by full-wave rectifying a commercial AC power source is known (hereinafter, referred to as an LED lighting device). This LED lighting device includes an LED string in which a plurality of LEDs is connected in series so as to be capable of resisting a high voltage, and has a threshold value and when the threshold value is exceeded, a current flows through the LED string and the LED string lights up. This threshold value is set to a value somewhat lower than the peak of the pulsating voltage (about 140 V), and therefore, when the effective value of the commercial power source is 100 V, the threshold value is set to about 100 to 120 V. Each of the LEDs has a threshold value called a forward voltage Vf and when a voltage equal to or higher than the forward voltage Vf is applied, a current flows and the LED lights up. The threshold value of the LED string is the sum of the forward voltage Vf of each LED included in the LED string.
When a pulsating voltage is simply applied to the LED string, the LED string lights up only for a period of time during which the pulsating voltage exceeds a threshold voltage. Thus, the LED string becomes dark and flickering becomes conspicuous and further, the power factor and the distortion factor also deteriorate. If the number of LEDs connected in series in the LED string is reduced to shorten the non-lighting period of time, the power loss of a current limiting circuit inserted in series with the LED string becomes large, and therefore, this is not preferable. Thus, there is proposed an LED lighting device intended to solve the above-described problems by switching the numbers of LEDs connected in series in the LED string caused to light up in accordance with a voltage applied to the LED string or a current flowing through the LED string (for example, Patent Documents 1, 2).
In FIG. 1 of Patent Document 1, a light-emitting diode lighting device (LED lighting device) is described, which adjusts the number of light-emitting diodes 14 (connected in series) caused to light up by dividing a light-emitting diode circuit 15 (LED string) into six diode circuits 17 to 22 and switching drive switches 30 to 35 based on a pulsating voltage.
In a circuit in which current paths are switched based on the pulsating voltage as in Patent Document 1, the current flowing through the LED string is reduced or increased considerably at the instant when the paths are switched. In other words, the current value becomes discontinuous and this causes various problems, such as an increase in harmonic noise. In contrast to this, in the LED drive circuit illustrated in FIG. 26 of Patent Document 2, by measuring the current flowing through the LED string, when the current exceeds a predetermined value, the number of LEDs connected in series in the LED string is increased and at the same time, the current is also increased continuously.
The circuit of FIG. 26 of Patent Document 2 is explained briefly (see FIG. 8). In FIG. 8, there is an LED string including an LED group 1, an LED group 2, and an LED group 3. When the current flowing through the LED string is small, a FET Q1 bypasses the current flowing through the LED group 1 and the LED group 2 and no electric flows through the LED group 3 (the LED group 3 does not light up). When the current increases, the circuit operates so that the sum of the current flowing through the FET Q1 and the current flowing through the LED group 3 is constant. At this time, the LED group 3 lights up faintly. When the current increases further and exceeds a predetermined value, the FET Q1 cuts off and all the currents flow through the LED group 3 and the LED group 3 lights up fully together with the LED groups 1 and 2. When the current decreases, the reverse operation is performed. The upper limit of the current is limited by a current limiting resistor R1.
When the LED is caused to light up in the circuit illustrated in FIG. 26 of Patent Document 1 illustrated in FIG. 8, if the pulsating voltage becomes high, the current flowing through the LED string also increases and if the pulsating voltage becomes low, the current flowing through the LED string also decreases, and therefore, there is an advantage that the power factor and the distortion factor are excellent.    Patent Document 1: JP-458646 (FIG. 1)    Patent Document 2: WO2011/020007 (FIG. 26)